Nodal Multiple Islands Logic

The nodal multiple islanding logic is activated by the Allow Multiple Islands in Base Network switch on the Nodal Study Type form in Simulation Options. This switch offers the ability to include all buses in a system with multiple islands during the nodal OPF solve and all branches outages that cause islanding are allowed to be part of a nodal simulation. Every dispatch hour, Aurora checks to see if new islands are created by the set of active branch outages for the hour. Each island is optimally dispatched independently, and any contingencies that do not introduce additional islands beyond those created by branch outages will also be used in the SCOPF dispatch.

When the Allow Multiple Islands in Base Network switch is selected, Aurora will perform the following checks every hour:

Verify if the system’s island structure has changed since the last dispatched hour. Active branch outages for the hour are considered while performing this check.

If the island structure has changed,
1. Slack Bus Selection: Aurora chooses a Slack Bus in each island based on the BusType input in the LoadFlow_Bus table in the Nodal Case file. If no bus in an island has been designated as Slack Bus, then Aurora chooses the bus with the largest PMax value as the slack bus.
2. Shift Factors Calculation: Aurora will re-compute the shift factors for each island.
If the island structure has not changed, then Aurora calculates the Line Outage Distribution Factors (LODF) for outage branches as needed. The shift factors are updated based on the calculated LODFs.
After the shift-factors for the N-0 networks have been updated based on the current hour’s island structure, Aurora evaluates all specified contingencies for post-contingent island structure. Only contingencies that do not cause further islanding are enabled for the current hour. LODFs are computed for valid contingencies.
Utilizing the appropriate shift factors, Aurora formulates the dispatch problem.

Angles Solution method: Aurora formulates the DC Optimal Power Flow (OPF) equation explicitly utilizing bus angle variables and network admittance matrix (Ybus matrix). Appropriate flow constraints and contingency flow constraints are added to the OPF formulation in the standard iterative fashion to formulate the Security Constrained OPF (SCOPF). The solution of the SCOPF is provided for the dispatch solution for the hour as well as Locational Marginal Pricing (LMP).
Shift Factor method: Aurora adds a supply balance constraint for each island. Appropriate flow constraints and contingency flow constraints are added to the OPF formulation in the standard iterative fashion formulate the Security Constrained OPF (SCOPF). The solution of the SCOPF is provided for the dispatch solution for the hour as well as LMP.

NOTE: The nodal Multiple Island logic can experience slower runtimes because shift factors and LODF’s need to be recalculated every time network island structures change due to branch outages.

Simulation Logic

Nodal Multiple Islands Logic